Brain and central nervous system tumors are often particularly difficult to treat. They may originate in the central nervous system (CNS) as primary tumors, or they may be metastases from tumors which arise in other organs or tissues. The most common types of primary tumor of the brain and CNS are malignant gliomas, which can spread aggressively and often diffusely into normal brain tissue.
Chemotherapy-based treatment of brain and central nervous system tumors is frequently ineffective, particularly in the case of metastases from non-primary tumors, which are often drug resistant. Furthermore, the blood brain barrier (BBB) presents a challenge to treatment by preventing access of therapeutic drugs to the tumor. As a result, the mean survival time for patients with metastases from small-cell lung cancer, breast cancer, and melanoma can be depressingly short. The addition of whole-brain radiation therapy can provide some added benefit, but such benefit is often minimal.